Ouyen, in Victoria's north-west, reached a top of 45.8 degrees this afternoon, the hottest November day ever in Victoria.

Temperatures also rose above 45 degrees in Mildura and Swan Hill.

The extremely hot conditions surpass the previous record of 45.0 degrees set in the early 1900s, also in the state's north-west.

Senior forecaster Dean Stewart says it is unusual the November record has stood for more than a century.

"In these days when we seem to have an increased number of hot days, I guess for a record to last that long is a little surprising, but it had to be passed at some stage and today's the day," he said.

Mr Stewart says a cooler change should reach Melbourne in the afternoon.

"There is a weak change pushing up towards Airey's Inlet," he said late this afternoon.

"It will be coming through, reaching the city area a bit after 9pm, so between 9pm and about 11pm, through the city.

"But it is pretty weak so there is not going to be too much relief by the time you get into the outer suburbs."

Earlier today the Country Fire Authority (CFA) placed extra aircraft in the north-west of the state as a precaution.

CFA spokesman Brett Boatman is asking farmers in the west and the north-west to delay harvesting until the danger passes.

"We're saying to harvesters and contractors if they can delay their works until the fire danger passes, that reduces the ignition and the chance of fires," he said.

The hot weather is also expected to cause delays on V/Line train services. Speed restrictions will be introduced between noon and 8:00pm (AEDT).

V/line says services this afternoon could be up to 30 minutes slower and may be replaced by buses.

'Pulling out all stops'

In South Australia, it is the first time in two years there are bans in force for every district, including metropolitan Adelaide.

Temperatures reaching the mid-40s were expected in some areas and strong winds had been forecast.

Authorities were also worried about possible lightning strikes later in the day and SA Power Networks is warning people need to be prepared for their electricity to be turned off if there is a danger as the conditions worsen.

Brenton Eden from the Country Fire Service (CFS) is expecting a challenging day.

"We will rely heavily on the public doing the right thing, reporting any fires that do start and behaving themselves according to the restrictions of a total fire ban," he said.

The worst of the weather passed through this morning, with a change in the weather bringing some relief this afternoon.

"The wind change is about three hours ahead of where it was predicted yesterday, so our major fire weather will be this morning," he said.

"But the concern then is if we get any fire starting, as we have seen in South Australia in the last two weeks, some of these fires can burn for weeks. The risk is if we get an ignition."